1. Field of the Invention
This invention relates to array antennas and, more specifically, to the side lobe patterns generated by those antennas.
2. Description of the Related Art
Conventional monopulse corporate-fed array antennas produce very high difference pattern side lobes in their principal planes. Referring now to FIG. 1, graph 10 shows an example of a sum pattern 12 and difference pattern 14 of a conventional corporate-fed standing wave array antenna. The sum pattern 12 has low sum pattern side lobes 16. The difference pattern 14 has relatively high side lobes 18. Referring now to FIGS. 2 and 3, the array aperture sum amplitude distribution 20 and difference amplitude distribution 22 are usually optimized for low sum pattern side lobes 16. The optimization creates an abrupt change at the center 24 of the difference amplitude distribution 22. It is this abrupt change, or discontinuity of the amplitude distribution, that produces the very high difference pattern side lobes 18.
There are existing approaches for achieving both low sum and difference side lobes from corporate-fed arrays. The Antenna Engineering Handbook, Third Edition, by Richard Johnson, FIG. 20-44 ("Johnson") discloses a typical existing approach comprising a phase monopulse corporate-fed phased array that produces low sum and difference side lobes. Johnson discloses the pairing of radiating elements that are symmetrically opposite from the centerline of the array. The paired radiating elements are combined in a magic T to form the sum and difference patterns. This approach is relatively complex and not applicable to corporate-fed wave guide standing wave array antenna design.
Therefore, a radar system comprising an array antenna of relatively simple construction that provides low difference side lobes is needed.